refactor: centralize Windows platform checks #157

Context

Currently, Windows detection logic is duplicated across multiple modules using checks such as platform === "win32" or process.platform === "win32".

As pointed out in the review discussion for PR #152 (#152 (comment)), these can be consolidated into a common helper function to be used across GitHub Actions and portable CI runtimes.

This change aims to be a refactoring that fully maintains existing behavior. To keep platform-dependent code easy to test, callers that take an explicit platform value should continue to pass that value to the helper function.

I will handle this.